Electronic image sensors such as charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) sensors typically include one or more photodiodes for each pixel that, when exposed to light, accumulate electrical charge in proportion to the brightness of the light incident on the photodiode of the pixel. Some electronic image sensors are used with mechanical shutters to help control the duration of time that the photodiodes are exposed to a light source, whereas other electronic image sensors (including certain of those on, for example, cell phones, tablet computers, and other electronic devices) are not used with a mechanical shutter and hence rely on some form of electrical shuttering to control the charge generated in the photodiodes when exposed to light.
One way in which electrical shuttering can be implemented is by controlling gates (e.g., transistors) that couple the photodiode to other parts of the sensor. For example, gates coupled to the photodiode may continuously drain away any charge generated in the photodiode when no image is to be captured by the sensor. Controlling the flow of charge generated in the photodiode may generally be referred to as exposure control because it controls the effective exposure of the photodiode, even though without a mechanical shutter, the sensor may constantly be exposed to light. In other words, because of the difficulty of mechanically controlling exposure of photodiodes, the effective exposure of some electronic image sensors may be controlled by selectively accumulating and selectively discarding charge generated by incident photons in those areas.
It can be useful to control exposure of pixels in an image sensor based on different lighting and color conditions of a particular scene to be imaged. For example, in a bright light situation, the exposure time of pixels on the sensor may be reduced to avoid saturating the wells of the photodiodes of the pixels (which, if not corrected, may lead to blooming). Conversely, in a low light situation, the exposure time may be increased in order to allow sufficient charge to collect in the pixel to maintain a sufficient signal-to-noise ratio. However, changing exposure times may be challenging for several reasons, particularly for image sensors that capture video at a set frame rate and thus have limited amount of time within which to expand exposure time. Continuing with the video example, on one hand, if the exposure time is shortened too severely within each video frame, severe motion artifacts may occur between frames of the video causing aliasing or a stroboscopic effect between frames. If, on the other hand, exposure time is lengthened too much, pixels may saturate, and may even bloom into adjacent pixels.
Color image sensors typically include some type of color filter array (CFA), such as a Bayer pattern having a red-green-blue-green (RGBG) arrangement of pixels. For a Bayer pattern, the pixels of the image sensor may be arranged into groups of 2×2 sub-arrays, with each 2×2 sub-array including one red pixel and one green pixel in one row, and one blue pixel and one green pixel in the other row. In such an arrangement, the two green pixels are generally located diagonally from one another in the 2×2 sub-array. Usually, all four pixels in each 2×2 sub-array are identical other than a color filter placed over the top of the pixel; in other words, the pixels usually have photodiodes with similar well capacity. However, the color filters cause different wavelengths of light to be detected by the different color pixels. The sensitivity of the photodiodes of the pixels to incident photons may, however, vary based on the wavelength of light incident thereon, and so different colored pixels may have different sensitivities to identical brightness of different colors of light. For example, in the Bayer pattern, the green pixels are usually the most sensitive. Because the green pixels are the most sensitive, the green photodiodes will typically fill up with charge faster than the red and blue photodiodes for most neutral color scenes to which the image sensor is exposed.
Recently there has been interest in adding a clear pixel, which may not have a colored filter placed over the top of it. The clear pixel is also known as a “white pixel.” Adding clear pixels to an image sensor may provide a higher luminance sensitivity and wider overall spectrum for the image sensor. However, the clear pixel is usually more sensitive than any of the red, green, or blue pixels to incident light, and thus the clear pixel typically saturates faster than any of the other color pixels.
One technique to help prevent image sensor pixels from saturating and potentially spilling accumulated charge over into adjacent pixels (a phenomenon known as blooming) is to drain excess charge away from the photodiodes using a form of anti-blooming control.
In most image sensors today, exposure control and anti-blooming are applied in a generally uniform manner. For example, most image sensors set a single, common exposure time for all of the different colors of pixels, and the exposure time is a continuous block of time. The exposure time is generally chosen responsive to the most sensitive color channel, with the exposure time being shorter than the time during which the most sensitive color pixels are expected to saturate with accumulated charge. However, cutting short the effective exposures of the other, less-sensitive color pixels may cause less than optimal results (possibly with low signal to noise ratios for those colors, as signal to noise ratio is generally related to the amount of charge accumulated in the photodiode). Furthermore, the uniform application of anti-blooming control among all of the different colors of pixels also prevents more fine-tuned blooming mitigation techniques from being applied.